Many MEMS sensors include a mechanical oscillator element. For example, MEMS gyroscope and/or accelerometer type sensors often include one or more proof masses, tuning forks or other oscillating structures that are electrostatically driven at a resonance frequency. Movements of the sensor housing, such as rotational movement, lateral movement, acceleration, or other movement can then be detected by sensing certain behavior in the oscillating structure. For example, the oscillating structure may move in a direction that is perpendicular to the oscillating direction due to externally applied forces, such as coriolis forces, acceleration forces, or other forces, depending on the application.
The operational performance characteristics of some MEMS sensors, such as MEMS gyroscope or MEMS accelerometer type sensors, are often related to the resonator Quality value (Q) of the sensor. For example, the start-up time of the mechanical oscillator element, the ring-down time, the sensitivity of the sensor, as well as other performance characteristics are often affected by the Q value of the sensor. The Q value of the sensor is dependent on a number of factors, including the overall sensor design.
Known dampening mechanisms within the sensor can affect the Q value of the sensor. One known dampening mechanism is dependent on the energy lost due to collisions of the mechanical oscillator element with gas molecules within the sensor cavity of the sensor package. To reduce this dampening mechanism, and to obtain higher Q values, such sensors are often packaged in a sensor cavity that is under low pressure. Such sensor packages are often referred to as vacuum packages, even though an absolute vacuum may not be used.
The packages for many MEMS sensors often do not have perfect seals, which results in gas leakage into or out of the sensor cavity. Over time, these leaks can change the internal package pressure, and thus may affect the Q value of the sensor. In some cases, a relatively small leak can cause a relatively large change in pressure in the sensor cavity, particularly over long periods of time. For some applications, this can cause the sensor to cease to operate in accordance with required design parameters after a certain period of time.
Recently, there has been an increased demand for MEMS sensors that have an extended useful life, such as 15 to 20 years. For these and other applications, a MEMS sensor must have a small enough leak rate so that the pressure in the sensor cavity does not exceed some pressure limit over the expected lifetime of the sensor. Currently, conventional methods for testing leak rates of sensor packages are in the 5×10−12 He atm.cc/s range, which is often not sensitive enough to test sensor packages with expected lifetimes of 15 to 20 years. Therefore, there is need for improved methods and systems for detecting leaks in packages that house MEMS sensors, and in particular, MEMS sensors that have a mechanical oscillating element.